1. Field of the Invention
The present invention relates generally to photorefractive glasses, and more specifically to photorefractive glasses for use as glass-based optical elements having a refractive index pattern formed therein.
2. Technical Background
Diffractive optical elements find use in a wide variety of fields. For example, diffractive optical elements are useful for filtering, beam shaping and light collection in display, security, defense, metrology, imaging and communications applications.
One especially useful diffractive optical element is a Bragg grating. A Bragg grating is formed by a periodic modulation of refractive index in a transparent material. Bragg gratings reflect wavelengths of light that satisfy the Bragg phase matching condition, and transmit all other wavelengths. Bragg gratings are especially useful in telecommunications applications; for example, they have been used as selectively reflecting filters in multiplexing/demultiplexing applications; and as wavelength-dependent pulse delay devices in dispersion compensating applications.
Bragg gratings are generally fabricated by exposing a photosensitive material to a pattern of radiation having a periodic intensity. Many photosensitive materials have been used; however, few have provided the desired combination of performance and cost. For example, Bragg gratings have been recorded in germanium-doped silica glass optical fibers; while such gratings are relatively robust, the fiber geometry and high melting point of the material make these gratings inappropriate for many optical systems. Bragg gratings have also been recorded in photorefractive crystals such as iron-doped lithium niobate. These filters had narrow-band filtering performance, but suffered from low thermal stability, opacity in the UV region, and sensitivity to visible radiation after recording. Photosensitive polymers have also been used as substrates for Bragg gratings; however, devices formed from polymeric materials tend to have high optical losses and high temperature sensitivity.
Photosensitive glasses based on the Ce3 +/Ag+ redox couple have been proposed as substrates for the formation of diffractive optical elements. For example, U.S. Pat. No. 4,979,975 (Borrelli) discloses a photosensitive glass containing, in weight percent on the oxide basis, about 14-18% Na2O, 0-6% ZnO, 6-12% Al2O3, 0-5% B2O3, 65-72% SiO2, 0-0.2% Sb2O3, 0.007-0.04% Ag, 0.008-0.005% CeO2, 0.7-1.25% Br and 1.5-2.5% F. In these materials, exposure to radiation (λ˜366 nm) causes a photoreduction of Ag+ to colloidal Ag0 and Ce3+ to Ce4+, which acts as a nucleus for crystallization of a NaF phase in a subsequent heat treatment step. These glasses had very high absorbances at wavelengths less than 300 nm, making them unsuitable for use with commonly used 248 nm excimer laser exposure systems.
More recently, disclosed in U.S. patent application Ser. No. 2002/0045104 (Glebov et. al.) is a NaF-based photosensitive glass that by the appropriate exposure and thermal development, produces a refractive index change in the near infrared that accompanied the development of the NaF phase; the glass composition falling within that composition described above in the Borrelli reference. This effect opened the possibilities for applications to optical.
Devices based upon a photorefractive effect, with examples including Bragg gratings and holographic elements. The specific composition disclosed by Glebov et al was very similar to that of the original Corning (Stookey et al). As disclosed above, the composition the important constituents are the concentrations of Ce+3 (photosensitizer), Ag+ (photonucleus), and F, with the latter controlling the amount of NaF that can be produced and consequently the maximum amount of possible induced refractive index change. In order to achieve the photosensitive/photorefractive effect in the glass Glebov's process, like the above described Borrelli reference involved the exposure to light in the vicinity of 300-nm, or greater, followed by a heat treatment of 520 C for 2 hours.
It would be beneficial, and is thus an objective of this invention to be able to produce photosensitive glasses, and thus optical elements, as a result of exposure to industry standard 248 nm wavelength (KrF excimer laser). The reason for this is that the exposure methods, equipment, capability, and reliability and know-how utilizing 248 nm excimer laser exposure to make accurate optical devices is already in place throughout the telecommunication industry. This has come about because of the wide use of fiber Bragg gratings which are fabricated in this manner.